It is known to provide fluid-working machines, such as pumps, motors and machines which operate as either a pump or a motor, which include a plurality of working chambers of cyclically varying volume, in which the flow of fluid between the working chambers and one or more manifolds is regulated by electronically controlled valves. Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas.
For example, fluid-working machines are known which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine. For example, EP 0 361 927 disclosed the method of controlling the net throughput of fluid through a multi-chamber pump by operating and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to either undergo an active cycle and displace a predetermined fixed volume of fluid, or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand. EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid-working machine which functions as a motor or which functions as either a pump or a motor in alternative operating modes. EP 1 537 333 introduced the possibility of part active cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand. By an idle cycle we refer to a cycle of working chamber volume where there is substantially no net displacement of fluid. Preferably, the volume of each working chamber continues to cycle during idle cycles. By active cycle we refer to any cycle of working chamber volume other than an idle cycle, where there is a predetermined net displacement of fluid, including part active cycles (e.g. part pump or part motor cycles) where there is a net displacement of a volume of fluid which is less than the maximum volume of fluid that the working chamber is operable to displace. Idle and active cycles may be interspersed, even at constant demand.
Fluid-working machines of this type require rapidly opening and closing electronically controllable valves capable of regulating the flow of fluid into and out of a working chamber from the low pressure manifold, and in some embodiments, the high pressure manifold. The electronically controllable valves are typically actively controlled, for example, actively opened, actively closed, or actively held open or closed against a pressure differential, under the active control of the controller. Although all opening or closing of an actively controlled valve may be under the active control of a controller, it is usually preferable for at least some opening or closing of the actively controlled valves to be passive. For example, the actively controlled low pressure valve disclosed in the fluid-working machines described above may open passively when the pressure in a working chamber falls below the pressure of the low pressure manifold, but be optionally actively held open to create an idle cycle or actively closed during a motoring cycle, just before top dead centre, to build up sufficient pressure within the working chamber to enable the high pressure valve to open.
An active cycle or an idle cycle may result from the active control of the electronically controllable valves. An active cycle or an idle cycle may result from the passive control of the electronically controllable valves.
In the event that one or more working chambers of a fluid-working machine comprising a plurality of working chambers become unavailable, for example if a fault occurs in one or more working chambers or in the control of one or more working chambers, the function of the fluid-working machine is dramatically impaired.
FIG. 1 shows a graph of the fluid pressure as a function of time at an output port of a fluid-working machine comprising six working chambers, operating as a pump to pump fluid through a hydraulic motor driving a vehicle. The six working chambers are piston cylinders slidably mounted to the same eccentric crankshaft such that their phases are mutually spaced apart by 60°. The machine includes a pressure accumulator to smooth the output from the individual working chambers. The machine comprises a controller which is operable to select the valve firing sequence in order to meet the demand signal.
Between time A and time B, the fluid working machine is functioning normally and the output pressure remains approximately constant in response to a constant displacement demand signal (corresponding to a constant vehicle speed) and valves are fired according to the method outlined in EP 0 361 927. The fluid-working machine executes a pattern of working chamber activations that repeats every five revolutions. The trace of output pressure with time shows both a fast pressure oscillation due to the fluid delivery by the individual activated working chambers, and a slow oscillation due to the short term average flow delivered by the activated working chambers being at times slightly above and at times slightly below the average flow required to maintain the same vehicle speed.
At time B, one of the six working chambers was deactivated, in order to simulate a malfunction in that working chamber. Between time B and time C, in response to the same demand signal, the output pressure initially drops dramatically when the controller causes the machine to try to activate the disabled working chamber. In response, the vehicle slows down, so when the controller returns to that part of the repeating pattern that does not use the deactivated working chamber, there is an excess of flow and a pressure overshoot. The cycle repeats each time an attempt is made to activate the disabled working chamber.
Thus, known fluid-working machines, which, in the event of the unavailability of one or more working chambers, issue output signals to meet a demand signal as though all of the working chambers were available, fail to function adequately when a working chamber is unavailable.
Therefore, there remains a need for a method of operating a fluid-working machine which mitigates this problem, and a need for fluid-working machines which perform better when a working chamber becomes unavailable.
Some aspects of the invention address the problem of identifying, confirming or diagnosing a fault in a fluid-working machine.